High performance voltage regulators are typically used in applications having large and fast-changing current load conditions such as when a memory device or processor operates in an active mode. High performance voltage regulators conventionally include an amplifier for generating a regulated voltage output in response to a reference voltage and a feedback voltage. Also included are a power transistor and bias network. The power transistor boosts the amplifier output while the bias network provides bias voltages to the amplifier for setting the internal bias currents of the amplifier. A high amplifier bias current allows quick regulation of the power transistor gate voltage, thus increasing regulator performance.
High performance voltage regulators are at least partially disabled when load currents are low and steady to reduce power consumption, e.g., during low power or standby mode. Power consumption is reduced when a voltage regulator is disabled because amplifier bias current is significantly reduced. One conventional approach for disabling a voltage regulator is to set the gate-to-source voltage of the power transistor to zero volts, thus turning off the power transistor. A switch may also prevent current flow through the bleeder resistor coupled to the power transistor. The regulator amplifier is also disabled by disconnecting the main bias voltage applied to the bias network, thus disabling the bias network. Each output node of the bias network is driven to an appropriate voltage level when the bias network is disabled, ensuring that the amplifier is properly disabled. This way, the bias voltages applied to amplifier do not float to problematic levels when the regulator is disabled.
When the voltage regulator is subsequently re-enabled, the bias network charges the internal capacitance of the amplifier from a disabled state to a desired level before the amplifier can generate a properly regulated output. Some conventional voltage regulator circuits include a startup circuit, such as boost capacitors, for assisting the bias network in setting the amplifier bias current when the regulator is being re-enabled. Conventional startup circuits are reset to an initial state when the voltage regulator is disabled. This way, the startup circuit is ready to assist the bias network when enabled, as long as the startup circuit was properly re-initialized while the voltage regulator was disabled.
However, voltage regulators can be disabled and then quickly re-enabled. If the regulator is re-enabled too quickly, conventional startup circuits may not have enough time to properly re-initialize while the voltage regulator is disabled. An improperly reset startup circuit may charge/discharge the amplifier bias voltages to problematic voltage levels, thus causing the amplifier to operate improperly. Improper amplifier operation may degrade circuit performance, cause circuit malfunction, and decrease yields. Further, conventional regulator amplifiers may not be properly disabled when regulator re-enablement occurs too quickly. For example, the amplifier bias voltages may not have enough time to fully charge/discharge to the appropriate level before the regulator is re-enabled. An improperly disabled amplifier may also cause performance degradation, malfunction, and decrease yields.